First off, thanks for the great reply! My point isn't that it's not useful as much as that it's very preference-based. The good news is though that apparently both Linux (GNOME + KDE) and macOS support this feature, despite my not using it much on any platform, so it's what we're discussing is not even a difference between mac/lin at this point, but just an abstract consideration.

> Being able to check the date or bottom line on a received PDF invoice, without opening it.

I tried double-clicking on a 43-page PDF, and it took less than a second to open on my under-powered Acer (about ~$200, mfg 4 years ago), running stock Ubuntu 20.04. This seems really fast, as it was basically instantaneous from releasing the mouse. Obviously opening up a PSD would be slower, but then again I can always just zoom in on the thumbnail in that case (Ctrl+Scroll zooms in on icons in Linux).

This is making me think perhaps the real difference is that the default programs that open are too slow, eg if Photoshop is the default associated application for all images, or Adobe Acrobat for all PDFs, then I totally would see the need to have a previewer as well, since last time I used those they were realllllly slow.

I dunno, that said, it's all really not important. Then again, what is HN but for getting grumpy about unimportant things? :p